554 MESSRS. HILL & KYNASTON ON KENTALLENITE [Aug. I9OO, 



by Teall, Harker and others, in our own country. We have a 

 variety of separate intrusions, of the same geological age, ranging 

 from basic to acid and forming a closely connected assemblage, 

 or, in other words, a series of rocks from independent mag- 

 matic eruptions which show also a continual progression or 

 gradation. In addition to this, we have also described a case of a 

 similar kind of gradation in a single intrusion, in which granite 

 passes marginally into augite-diorite. A progressive series of this 

 latter type has been termed a facies-suit; and we may cite as 

 typical examples of this the shonkinite-laccolites of Square Butte 

 and Yogo Peak, Montana, described by Messrs. Weed & Pirsson. 

 What is true of a facies-suit on a comparatively small scale, 

 is also true of a series of separate intrusions on a larger scale. 

 Owing to the remarkable parallelism in the two phenomena, it is 

 possible that both owe their present petrological characters to a 

 similar kind of magmatic differentiation, which in the one case 

 has taken place within the limits of a single intrusion, and in the 

 other case has been controlled by more deep-seated conditions. In 

 both cases we have to deal with differentiation which has resulted 

 in the production of basic material marginally. To glance briefly at 

 one of the American examples, the Square-Butte laccolite, Messrs. 

 Weed & Pirsson x conclude that after the intrusion, differentiation 

 took place in the liquid mass, the iron-, magnesia-, and lime-molecules 

 being greatly concentrated in a broad exterior zone, leaving an inner 

 kernel of material richer in alumina, alkalis, and silica. This latter 

 crystallized into a sodalite-syenite, while the outer mass formed the 

 basic rock to which the name shonkinite has been given. The 

 mass — shonkinite 4- soda-syenite — forms a geological unit. Differ- 

 entiation, according to the above-mentioned authors, took place 

 in situ, and was regulated by the diffusion of the basic oxides to 

 the outer cooling surface. This idea is apparently confirmed by 

 Prof. Brogger's conclusions with regard to the eruptive rocks of Gran 

 in Norway 2 and in other areas, and by Mr. Harker's studies of 

 the Carrock-Fell gabbro. 3 It seems not unreasonable therefore to 

 conclude that a similar type of differentiation is illustrated in the 

 Ben-Bhuidhe area. Thus, in the case of the single intrusion on 

 Beinn Chas, showing basic material developed marginally to more 

 acid, we may suppose that a similar kind of differentiation took place 

 in the magma subsequently to intrusion, resulting in the production 

 of more basic material surrounding a more acid core. That the 

 differentiation in the case of the various Ben-Bhuidhe intrusions, 

 ranging from granite to kentallenite, was also of a marginal cha- 

 racter, the distribution of the different rock- masses sufficiently 

 shows. On the other hand, it must have taken place in the parent- 

 magma of these intrusions under much more deep-seated conditions, 

 our knowledge of which is necessarily of a strictly limited nature. 

 But whatever may have been the determining factor in the process, 



1 Bull. Geol. Soc. Am. vol. vi (1895) p. 422. 



2 Quart. Journ. Geol. Soc. vol. 1 (1894) p. 15. 



3 Ibid. p. 311 & vol. li (1895) p. 125. 



